This invention relates to a method of and apparatus for generating power, and more particularly, to a combined cycle, gas turbine-based power plant, and to a method for generating power therefrom.
Combined cycle, gas turbine-based power plants are becoming popular today throughout the world because of their high efficiency, reliability, and ruggedness. Conventionally, such power plants include a gas turbine unit having a compressor for compressing ambient air and producing compressed air, a combustion chamber to which the compressed air is supplied, a source of fuel for burning in the combustion chamber and producing combustion gases, and a gas turbine connected to and driving a generator and the compressor for expanding the combustion gases and producing exhaust gases.
The apparatus further includes an energy converter which uses water as a working fluid and which is responsive to the exhaust gases of the turbine for converting heat in the exhaust gases to electricity. The cooled exhaust gases are then vented to the atmosphere.
The use of water as the working fluid of the energy converter, and the presence of a steam turbine, results in a complex power plant operating on high pressure steam which adversely affects operating expenses of the power plant. For example, the plant must be manned at all times by a licensed steam engineer; and a water treatment system is required. Condenser vacuum problems must be taken into account, and special provision must be made for winter operation when the temperature is below freezing.
It is an object of the present invention to provide new and improved apparatus, and method for using the same, for generating power which is less complex in construction and easier to maintain than the conventional apparatus described above.
Apparatus for generating power according to the present invention includes a gas turbine unit having a compressor for compressing ambient air and producing compressed air, a combustion chamber to which the compressed air is supplied, a source of relatively high grade fuel for burning in the combustion chamber and producing combustion gases, and a gas turbine connected to a generator and to the compressor for expanding the combustion gases and producing exhaust gases. The apparatus further includes a combustor which burns relatively low grade fuel and produces combustion products, and an indirect contact heat exchanger responsive to the combustion products for heating the compressed air before the latter is applied to the combustion chamber, and for producing cooled combustion products. In addition, heat in the exhaust gases of the turbine are utilized by feeding back these gases to the combustor, or applying them to an energy converter containing an organic working fluid that converts heat in the exhaust gases to electricity, and produces cooled exhaust gases which are directed to a stack, or optionally, to the combustor.
According to the present invention, the low grade fuel burned or combusted in the combustor comprises inexpensive fuel such as low grade coal, oil shale, petcoke, asphaltene refuse, or waste. In the latter case, the combustor may be an incinerator.
The use of an organic fluid instead of water as the working fluid of the energy converter permits unmanned operation of the power plant, and eliminates the need for water treatment, vacuum pumps for the condenser, and problems with freezing temperatures. Furthermore, the present invention permits a high total plant efficiency while at the same time achieves relatively low fuel consumption of high grade fuels, such as natural gas, kerosene, liquid petroleum gas, etc., used in the combustion chamber of the gas turbine. This is extremely important when the inexpensive fuel is a material which has to be disposed of such as waste, or solid waste, refinery residues, mine culm, etc.
Preferably, the energy converter includes a vaporizer containing an organic fluid that is vaporized by the exhaust gases of the gas turbine unit and which produces cooled exhaust gases, an organic vapor turbine responsive to vaporized organic fluid produced by the vaporizer for producing power and expanded organic vapor, and an organic vapor condenser responsive to the expanded organic vapor for condensing the latter to condensate. Means are provided for returning the condensate to the vaporizer.
Preferably, the organic fluid is selected so that the condenser pressure is close to, but above atmospheric pressure. Suitable working fluids are normal pentane and iso-pentane.
The invention also consists in means for accommodating the use of relatively low grade fuel burned in said combustor when the heating value of such fuel varies over time, which is a typical characteristic of low grade fuels. Apparatus according to the invention includes an element for generating a control signal related to the temperature of combustion gases produced by burning the relatively high grade fuel in the combustion chamber associated with the gas turbine, and control means responsive to the control signal for controlling the rate at which the relatively high grade fuel is burned in the combustion chamber. Such apparatus rapidly compensates for a reduction in heating value of the low grade fuel supplied to the combustor by rapidly increasing the rate at which high grade fuel is fed to the combustion chamber. Preferably, in addition, the apparatus also increases the rate at which the low grade fuel is supplied to the combustor in order to return the feed rate of the high grade fuel to its original value as quickly as possible. If the heating value of the low grade fuel increases, the apparatus of the invention is effective to reduce the feed rate of the high grade fuel. Ultimately, the apparatus of the invention serves to minimize the consumption of high grade fuel in the presence of changes in the heating value of the low grade fuel
In this manner, the temperature of the combustion gases are maintained at a substantially constant temperature for a given electrical load independently of changes in the heating value of the relatively low grade fuel burned in said combustor.
In a further embodiment of the invention, the low grade fuel is used directly for producing steam, and the exhaust gases from the gas turbine are used for superheating the steam, the excess heat in the exhaust gases being used for producing power using an organic Rankine cycle energy converter. This embodiment is particularly suitable for retrofitting an existing incinerator plant where the required maintenance of the steam system is tolerable.